Earlier studies have shown that amino acids and sugars are not uniformly distributed in the water of frog muscle, particularly at O degrees C. The exact location of metabolically useful substrates within these cells is not known. Studies proposed will use compartmental analysis of efflux curves to find the nutrient concentration in the extracellular space, the sarcoplasmic reticulum, and the sarcoplasm under various conditions including insulin-treated and non-insulin-treated muscle. Selective studies of incorporation of these nutrients into macro-molecules are planned to determine which of these compartments is most important to the metabolic utilization of the substances in question. Additional studies of the localization of amino acids within frog muscle are planned using osmotic techniques and the photoaffinity-labelled amino acid, p-azidophenylalanine.